(a) Field of the Invention
The present invention relates to a non-aqueous electrolyte and a lithium secondary battery comprising the same, and more particularly, to a non-aqueous electrolyte for a lithium secondary battery that prevents a battery from swelling, while maintaining the electrochemical properties of the battery.
(b) Description of the Related Art
Due to recent trends toward more compact and lighter portable electronic equipment, there has been a growing need to develop a high performance and large capacity battery to power portable electronic equipment. In particular, there has been extensive research to provide lithium secondary batteries with good safety characteristics and improved electrochemical properties. Lithium secondary batteries use materials that reversibly intercalate or deintercalate lithium ions during charge and discharge reactions for both positive and negative active materials. The positive active materials include lithium metal oxide and the negative materials include lithium metals, lithium-containing alloys, or materials that are capable of reversible intercalation/deintercalation of lithium ions such as crystalline or amorphous carbons, or carbon-containing composites.
The average discharge voltage of a lithium secondary battery is about 3.6 to 3.7V, which is higher than other alkali batteries, Ni-MH batteries, Ni—Cd batteries, etc. However, an electrolyte that is electrochemically stable in the charge and discharge voltage range of 0 to 4.2V is required in order to generate such a high driving voltage. As a result, a mixture of non-aqueous carbonate-based solvents, such as ethylene carbonate, dimethyl carbonate, diethyl carbonate, etc., is used as an electrolyte. However, such an electrolyte has significantly lower ion conductivity than an aqueous electrolyte that is used in a Ni-MH battery or a Ni—Cd battery, thereby resulting in the deterioration of battery characteristics during charging and discharging at a high rate.
During the initial charge of a lithium secondary battery, lithium ions, which are released from the lithium-transition metal oxide positive electrode of the battery, are transferred to a carbon negative electrode where the ions are intercalated into the carbon. Because of its high reactivity, lithium reacts with the carbon negative electrode to produce Li2CO3, LiO, LiOH, etc., thereby forming a thin film on the surface of the negative electrode. This film is referred to as an organic solid electrolyte interface (SEI) film. The organic SEI film formed during the initial charge not only prevents the reaction between lithium ions and the carbon negative electrode or other materials during charging and discharging, but it also acts as an ion tunnel, allowing the passage of only lithium ions. The ion tunnel prevents disintegration of the structure of the carbon negative electrode, which is caused by co-intercalation of organic solvents having a high molecular weight along with solvated lithium ions into the carbon negative electrode.
Once the organic SEI film is formed, lithium ions do not react again with the carbon electrode or other materials, such that an amount of lithium ions is maintained. That is, carbon from the negative electrode reacts with the electrolyte during the initial charging, thus forming a passivation layer such as an organic SEI film on the surface of the negative electrode such that the electrolyte solution no longer decomposes, and stable charging and discharging are maintained (J. Power Sources, 51 (1994), 79-104). As a result, in the lithium secondary battery, there is no irreversible formation reaction of the passivation layer, and a stable cycle life after the initial charging reaction is maintained.
However, gases are generated inside the battery due to decomposition of the carbonate-based organic solvent during the organic SEI film-forming reaction (J. Power Sources, 72 (1998), 66-70). These gases include H2, CO, CO2, CH4, C2H6, C3H8, C3H6, etc. depending on the type of non-aqueous organic solvent and negative active material used. The thickness of the battery increases during charging due to the generation of gas inside the battery, and the passivation layer slowly disintegrates by electrochemical energy and heat energy, which increase with the passage of time when the battery is stored at a high temperature after it is charged. Accordingly, a side reaction in which an exposed surface of the negative electrode reacts with the surrounding electrolyte occurs continuously. Furthermore, the internal pressure of the battery increases with this generation of gas. The increase in the internal pressure induces the deformation of prismatic and lithium polymer batteries. As a result, regional differences in the cohesion among electrodes inside the electrode assembly (positive and negative electrode, and separator) of the battery occur, thereby deteriorating the performance and safety of the battery and making it difficult to mount the lithium secondary battery set into electronic equipment.
For solving the internal pressure problem, there is disclosed a method in which the safety of a secondary battery including a non-aqueous electrolyte is improved by mounting a vent or a current breaker for ejecting internal electrolyte solution when the internal pressure is increased above a certain level. However, a problem with this method is that mis-operation may result from an increase in internal pressure itself.
Furthermore, a method in which the SEI-forming reaction is changed by injecting additives into an electrolyte so as to inhibit the increase in internal pressure is known. For example, Japanese Laid-open Patent No. 97-73918 discloses a method in which high temperature storage characteristics of a battery are improved by adding 1% or less of a diphenyl picrylhydrazyl compound to the electrolyte. Japanese Laid-open Patent No. 96-321312 discloses a method in which cycle life and long-term storage characteristics are improved using 1 to 20% of an N-butyl amine based compound in an electrolyte. Japanese Laid-open Patent No. 96-64238 discloses a method in which storage characteristics of a battery are improved by adding 3×10−4 to 3×10−2 M of calcium salt to the electrolyte. Japanese Laid-open Patent No. 94-333596 discloses a method in which the storage characteristics of a battery are improved by adding an azo-based compound to inhibit the reaction between an electrolyte and a negative electrode of the battery.
Such methods as described above for inducing the formation of an appropriate film on a negative electrode surface such as an organic SEI film by adding a small amount of organic or inorganic materials are used in order to improve the storage characteristics and safety of a battery. However, there are various problems with these methods. For example, the added compound decomposes or forms an unstable film by interacting with the carbon negative electrode during the initial charge and discharge due to inherent electrochemical characteristics, resulting in the deterioration of the ion mobility in electrons. Also, gas is generated inside the battery such that there is an increase in internal pressure, resulting in significant deterioration of the storage, safety, cycle life, and capacity characteristics of the battery.